There are many different types of recreational boats. These include outboards and inboard-outboards, sometimes known as stern drives. These boats typically have an internal combustion engine, either gasoline- or diesel-powered, arranged to drive a submerged propulsion system. These types of drives are typically water-cooled. More particularly, ambient water is drawn into the engine's cooling system, is circulated in heat exchange relation within the engine, and is discharged back into the ambient water. Thus, the effectiveness of such cooling systems depends upon the ability of water to circulate freely therewithin.
While many different types of marine life can enter and grow within, such cooling systems, particularly if the boat remains in the water but is used only intermittently, the problem of cooling system occlusion has recently been accentuated with the introduction of the zebra mussel into the Great Lakes and other waterways. Analogous problems are known to exist in salt water, where tube worms may attach themselves within conduits and passageways of marine cooling systems.
The zebra mussel, dreissena polymorpha, is a relatively-small bivalve mollusk with elongated thick shells marked by alternating light and dark bands. Thought to be native to the Black and Caspian Seas, the zebra mussel was reportedly introduced into European fresh water ports in the late 18th century. It proliferated during the next 150 years, and is now found throughout virtually all European inland waterways. Although the specific path of its introduction into North America is unknown, it is believed to have been transported from Europe in ships' ballasts, which were discharged into the Great Lakes. The zebra mussel was first discovered in Lake St. Clair in June of 1988. Having few natural enemies in North America, it has again proliferated, and is now found in all of the Great Lakes. The largest infestations are believed to presently be in the St. Clair River, Lake St. Clair, the Detroit River, Lake Erie, Lake Ontario, and the Erie Barge Canal. However, the ecosystem of the Great Lakes is favorable to the zebra mussel, and it is expected to continue to proliferate.
The zebra mussel will attach itself to almost any hard surface, and poses a substantial threat to public water supplies, lake ecology and recreation, such as swimming, fishing and boating. The threat of such barnacle-like colonization to submerged water intakes, pipes and conduits is well documented. Utilities are now devoting considerable time and attention to the removal or management of these colonies. Left unchecked, the incrustations can result in progressive constrictions of such flow passageways.
Zebra mussels can be transported to new areas as both larvae and as adults. The microscopic larvae, known as veligers, will readily pass through even fine-mesh screens. Mature female mussels can reportedly produce between 30,000 to 40,000 eggs per year in water temperatures of about 12.degree. C. (54.degree. F.). The veligers are reportedly capable of active swimming for one to two weeks following hatching, enabling them to travel for considerable distances from their parent colonies. Within three weeks of hatching, the young veligers reach the "settling stage", at which they attach themselves to submerged objects. These young mussels are capable of crawling along submerged objects at speeds on the order of 3.8 centimeters per hour until they find a suitable location at which to attach. The young mussels show an affinity of attaching to firm substrates in water currents of less than 2.5 meters per second. After attaching to the substrate, the young zebra mussels grow rapidly, and may reach an adult length of about 5 centimeters. The average life span is about 3.5 years, but can be as high as five years under ideal conditions.
Zebra mussels filter phytoplankton (i.e., microscopic plants and many forms of algae) from the water. It has been reported that an average single mature zebra mussel can filter about one liter of water per day. Since its recent introduction into the Great Lakes, some researchers have reported a two-fold increase in water clarity in mussel-infested areas. While this may appear to be desirable at first blush, by removing phytoplankton, the zebra mussel upsets the ecological balance in a lake or waterway, and threatens fish population. Some swimmers in infested areas are now reported to wear foot gear to avoid being cut by mussel shells.
The zebra mussel is also known to have a substantial impact on recreational boating. The mussels may attach themselves to the exterior surfaces of boat hulls, thereby increasing drag and reducing fuel efficiency. Such mussels may also enter the cooling system of recreational boats through submerged water intakes, particularly if a boat is tied to a dock or otherwise moored for long periods of time between use. Left unchecked, the mussels may progressively constrict the opening of various passageways, thereby impeding the intended free flow of cooling water through the cooling system.
Considerable research is being currently directed to the development of various techniques for controlling and removing mussel colonies. While the mussels attach themselves tenaciously to submerged surfaces, they may be physically scraped therefrom. Such scraping may be the preferred form of mussel removal from the exterior surfaces of boat hulls, but is somewhat difficult to effect in closed passages, such as the internal passageways of a cooling system.
It is also known that the mussels may be killed by oxygen deprivation. It is reported that exposure to anaerobic water at about 23.degree.-24.degree. C. (73.5.degree.-75.degree. F.) for two to three days will result in 100% mortality. Certain gases, such as hydrogen sulfide, may be added to increase the effectiveness of such oxygen deprivation techniques.
Other experiments involve the use of metallic ion control, and chemical control. Chemical control may be effected through the use of chlorinization, ozone, copper sulfate or the like. The problem here is that such chemicals are disfavored because of their toxic discharges into waterways.
It is also known that zebra mussels are sensitive to heat. It has been reported that zebra mussels will begin to die in water temperatures exceeding about 37.degree. C. (98.6.degree. F.) and that most mussels will be killed quickly by exposure to water temperatures of between 45.degree.-55.degree. C. (113.degree.-131.degree. F.) for a minimum of ten minutes. Moreover, upon application of heat of this order, the mussels tend to die with their shells slightly open, promoting exposure and degeneration of the byssal threads by which the mussels attach themselves to the substrate. It has been further reported that exposure to heat greater than about 60.degree. C. (140.degree. F.) results in virtually-immediate mussel mortality. Because of this, boaters have been advised to wash their boats, and flush their cooling systems, with hot water on the order of about 135.degree.-145.degree. F.
Because a source of hot water may not be readily available to many boaters, let alone to those who keep their boats in the water for extended periods, there is an immediate and pressing need for an improved method of killing marine life, such as zebra mussels, within the engine cooling system of such boats.